Acoustic wave devices using piezoelectric thin film resonators are used as filters and duplexers for wireless devices such as, for example, mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film. A region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region.
Rapid diffusion of wireless systems has promoted the use of many frequency bands. As a result, the filter and the duplexer have been required to have steep skirt characteristics. One way for making the skirt characteristic steep is increasing the Q-value of the piezoelectric thin film resonator.
Japanese Patent Application Publication No. 2006-109472 (Patent Document 1) discloses a piezoelectric thin film resonator including an annulus on the surface of one of the upper electrode and the lower electrode. U.S. Pat. No. 9,048,812 (Patent Document 2) discloses a piezoelectric thin film resonator including an annulus called a bridge in the piezoelectric film. Japanese Patent Application Publication No. 2003-505906 (Patent Document 3) discloses a piezoelectric thin film resonator including a ring-shaped recess on the upper surface of the upper electrode.
The piezoelectric thin film resonators disclosed in Patent Documents 1 and 2 can reduce the leak of the acoustic wave energy from the resonance region, and improve the Q-value. The piezoelectric thin film resonator disclosed in Patent Document 2 can reduce spurious. However, the degree of improvement of the characteristics of the piezoelectric thin film resonator and the reduction of spurious are insufficient.